The present disclosure relates to switching DC-to-DC converters.
Recent switching DC-to-DC converters exhibit highly-efficient power conversion characteristics and, therefore, are used as power supply circuits for electronic equipment in many cases. In particular, for such power supply circuits used for mobile equipment typified by cellular phones, it is necessary to reduce current consumption in a light-load state, e.g., a non-operating mode or a standby mode, in order to make batteries as power sources usable for a long period of time.
An example of such a DC-to-DC converter is disclosed in Japanese Laid-Open Patent Publication No. 10-248238. When a load driven by this DC-to-DC converter is stopped or suspended, the DC-to-DC converter increases the external resistance of an oscillator to reduce the frequency of a generated pulse signal, thereby extending the switching period. In the DC-to-DC converter, since switching losses proportional to the number of switching operations are dominant, such reduction of the switching frequency in a light-load mode minimizes power losses.
Japanese Laid-Open Patent Publication Nos. 64-39265 and 6-303766 disclose a method for minimizing losses by causing the DC-to-DC converter to perform burst operation, i.e., to alternatively perform operation and suspension and, thereby also suppressing current consumption in a control circuit in a long suspension period in a light-load mode.
However, in the DC-to-DC converter disclosed in Japanese Laid-Open Patent Publication No. 10-248238, control of an output voltage itself is unchanged from that in a normal operation, and the switching frequency is reduced by switching a constant of an oscillator. Since it is difficult to reduce the switching frequency to a few tenths or hundredths of that in normal operation, the above-mentioned method is not as good as the burst operation in terms of loss reduction. However, the burst operation has a drawback in which repetition of an operation period of high-frequency switching causes burst noise.